Test equipment has been developed to study the operation and performance of magnetic disk drives. Contact start-stop (CSS) testing traditionally uses force and acoustic data to study the characteristics and durability of the head/disk interface in disk drives. One of the purposes of this testing is to estimate the expected error rate of information written on the disk. The error rate is estimated by inference and extrapolation of the CSS force and acoustic data rather than from direct measurement.
As the read/write head moves over a disk surface, both the head and the disk wear, creating a variety of problems. The wear affects the error rate of data written on the disk surface and ultimately affects the lifetime of the disk drive. One of the most important purposes of CSS testing is to determine the increase in error rate with wear. The most direct way to accomplish this would be to write a data track, conduct the CSS test and compare the error rates on the written track before and after the test. However, CSS testing is conducted on disks which do not contain the servo track information that is later incorporated into disks to allow the read/write head to return to a previously written track. Without a servo track, the head cannot be positioned with the accuracy required to obtain a valid read signal. Therefore, it has not been possible to incorporate read/write testing into CSS wear testing without the use of extremely expensive micropositioning equipment.
Existing CSS testers can write a data track and then immediately read the data track while the read/write head is still aligned over the written track. After a short period of time, the signal is permanently lost due to movement of the read head because of environmental effects, such as vibration and temperature changes, or because of deliberate movement of the head as a part of common CSS tests protocols. Since CSS testers cannot return to read a previously written track, the degradation of the previously written data as a function of wear cannot accurately be determined. Because of this, existing CSS testers must write a new track after seek or sweep movements, and cannot directly determine the effect of wear on previously written data.
The inability of existing CSS testers to return to and recover data from a previously written track causes such testers to have significant limitations. A data track cannot be written and later accurately read in an area of the disk which does not undergo CSS wear testing. Writing and then reading back the information immediately thereafter can introduce potentially large variations in the read signal due to vibration of the head and/or flexure, and the fact that the data was written at a slightly different radial location. Accordingly, there is a need for testing methods and apparatus which overcome one or more of the above described drawbacks and disadvantages.